A number of printing techniques for applying markings to articles, including screen printing and transfer printing, are well known. These are generally surface printing methods which means that the identification mark realized can become unreadable through mechanical damage, abrasion, chemical influences, and the like. Such printing is particularly difficult to apply to curved or textured surfaces and the special tooling costs required add to the overall cost of the product.
The labeling of products is becoming of increasing importance in virtually all sectors of industry. Thus, for example, production dates, use-by dates, bar codes, company logos, serial numbers, etc., must frequently be applied. At present, these marks are predominantly made using conventional techniques such as printing, embossing, stamping, and labeling. However, the importance of non-contact, very rapid and flexible marking using lasers, in particular in the case of plastics, is increasing. This technique makes it possible to apply graphic inscriptions, for example bar codes, at high speed even on a non-planar surface. Since the inscription is in the plastic article itself, it is durable and abrasion-resistant.
The desirability of marking articles through the use of a laser system is well known. Lasers have been developed so that the beam impinged on the article to be marked can be highly focused to provide fine lines in the form of letters and/or numbers of the desired size, as well as images. Lasers permit the marking to be on the surface of the article or beneath the surface. In many instances, it is desirable to have the mark disposed subsurface in order to make it more difficult to remove the indication. Such a subsurface mark, can for example, contribute to anti-counterfeiting efforts. Laser marking, whether surface or subsurface, has also been used, for example, for electronically scanning and control purposes during production.
A number of laser beam processes in which an identification mark is burned into the surface of an article part are known. The resulting rough surface usually has to be coated with a transparent lacquer on account of the danger of contamination and the unpleasant feel that results. This can become a very involved operation in the case of mass produced parts and adds to the cost of the product.
The use of laser beam marking systems for creating subsurface marks is also known. Such systems are based on creating the mark by having the article to be marked be composed of a special configuration of materials or incorporating a material within the article which either becomes visible when exposed to the laser beam or causes something else present to become visible.
For example, U.S. Pat. No. 4,822,973 discloses a system in which the laser beam passes through the surface of a first plastic material in order to be absorbed in a layer of a second plastic material. This system requires a special configuration of materials of construction in the part to be marked. Other systems incorporate a quantity of carbon black, coated or uncoated “silicate containing” materials such as mica, talc, or kaolin, or a highly absorbing green pigment, all of which absorb energy from the laser beam to produce a visible mark. However, these materials have a degree of color or import a hazy quality to the plastic which is sufficient to be visible prior to application of the laser beam and that can be unsightly or interfere with the distinctness of the mark after the laser beam has been applied. This disadvantaging characteristic is aggravated by the fact that these additives tend to require a high loading content into the article to be marked, which is not only undesirable because of the effect on appearance but also can effect the physical and mechanical properties of the object. Further, absorbance of the laser beam to cause local heating also causes a degree of foaming which may detract from the creation of a fine and distinct dark mark, resulting in a blemished product.
Typically, the additives tend to be specific to the wavelength emitted by the laser. For example, laser marking materials initially developed for use in conjunction with carbon dioxide lasers tend not to work particularly well (or even at all) with the increasingly popular yttrium aluminum garnet (YAG) lasers which require a material which absorbs at 1064 nm.
In commonly assigned, U.S. Pat. No. 6,693,657, issued Feb. 17, 2004, a novel YAG laser marking additive (MARK-IT™) and its use is described. The YAG laser marking additive is a calcined powder of co-precipitated mixed oxides of tin and antimony. When the powder absorbs YAG laser energy and converts it into heat, carbonization of the surrounding material occurs and results in the formation of a black or dark mark that contrasts to the remainder of the surrounding area. Because of the particle size of the powder and its efficiency, the powder does not impart an appreciable amount of color to the object in which it is incorporated. It also does not cause excess foaming so that the mark achieved is smoother in texture.
Laser marking additives render polymers laser markable by acting as a light absorber for the laser light. Materials that act in this capacity often absorb visible light as well, which imparts a color to the piece to be marked. The color can be in contrast to the desired color of the piece, or it may dilute the desired color. The additive may also reduce clarity of a transparent piece. An appearance change can also be due to scattering of light by the additive. This can happen whether the additive has color or not. As a result, laser marking additives must be used in low concentrations, and/or not used in transparent applications.
It is therefore an object of this invention to provide a laser marking additive which will produce a black or dark mark contrasting with the surrounding area when exposed to laser energy but prior thereto does not impart an appreciable color to the surrounding area or cause a significant change in the performance of the material in which it has been added. Traditional additives cause transparent polymers to appear hazy, as such polymers lose optical clarity and are no longer crystal clear.
This invention produces a laser marking additive that does not change the color or appearance of articles at equivalent loadings to normal laser marking additives, and retains the laser markability of the piece. Moreover, optical clarity of clear and transparent polymers is maintained.